1. Field of the Invention
This invention relates to fluxing underfill compositions useful for fluxing metal surfaces in preparation for providing an electrical connection and sealing the space between semiconductor devices (such as chip size or chip scale packages (“CSPs”), ball grid arrays (“BGAs”), land grid arrays (“LGAs”), flip chip assemblies (“FCs”) and the like, each of which having a semiconductor chip, such as large scale integration (“LSI”)), or semiconductor chips themselves, and a circuit board to which the devices or chips, respectively, are electrically interconnected. More particularly, the present invention relates to underfill compositions capable of releasing a fluxing agent.
2. Brief Description of Related Technology
In recent years, the popularity of small-sized electronic appliances, such as camera-integrated video tape recorders and portable telephone sets, has made size reduction of large scale integration desirable. As a result, chip size or chip scale packages are being used to reduce the size of packages substantially to that of bare chips. Such chip scale packages include a semiconductor chip mounted on a carrier substrate, which improve the characteristics of the electronic device while retaining many of their operating features, thus serving to protect semiconductor bare chips and facilitate testing thereof.
Upside down integrated circuits, commonly referred to as “flip chips”, are now gaining popularity as well. Flip chips are manufactured using solder bump technology, in which solder bumps are deposited on solder-wettable metal terminations on a die or chip and a matching pattern of solder-wettable terminations on the substrate. With flip chips, the solder bumps are placed on the integrated circuit terminals while the chip is in wafer form, and then, after singulation, a chip is flipped and aligned to the circuit board substrate. A fluxing agent is applied and the solder bumps are re-flowed by heating to establish bonding between the chip and the substrate, with all the joints being made simultaneously by melting the solder. Typically, eutectic tin/lead solder (melting point 183° C.) or lead/indium solder (melting point 220° C.) are used.
After the solder reflow cycle, residue from the flux would typically be removed in order to prevent semiconductor device corrosion using organic- or aqueous-based solvents, depending on the nature of the flux. The narrow space between the semiconductor device and the substrate, however, renders flux residue removal difficult and time consuming, requiring sophisticated and expensive cleaning systems.
Moreover, when the resulting circuit board assembly is exposed to thermal cycling, the reliability of the solder connection between the circuit board and the chip often becomes suspect. Commonly, after a chip is mounted on a circuit board, the space between the chip and the circuit board is filled with a sealing resin (often referred to as underfill sealing) in order to reinforce against stresses caused by thermal cycling. Such underfill encapsulation has gained considerable acceptance in the electronics industry, with epoxy-based resin materials being most commonly used in such applications. Moreover, the expansion coefficients of the underfill sealing can be adjusted, for example, by the addition of low thermal-expansion fillers such as glass or ceramics, thus reducing the level of thermal stress that develops between the substrate and the underfill sealing. The underfill sealing thus provides structural reinforcement, which delocalizes the thermal expansion stress, thereby improving heat shock properties and enhancing the reliability of the structure.
Also, the underfill material helps adhere the chip to the substrate. As such, the underfill material should exhibit high cohesive strength to the die and the circuit board surface, and retain significant strength within the environment encountered by the electronic device, for example, during heat-up and cool-down cycles associated with on/off powering of the electronics, as well as climatic changes in temperature and humidity.
In an attempt to overcome fluxing residue issues and to eliminate processing steps, underfill sealants incorporating fluxing agents for bonding of the solder bumps have been proposed. For example, U.S. Pat. Nos. 5,985,043 and 5,985,486 disclose polymerizable fluxing agents which act as an adhesive to bond the chip to the substrate. Such polymerizable fluxing agents are based on polycarboxylic acids having olefinic linkages, compositions of which are curable upon exposure to heat. The thinking here is that the underfill sealant incorporating such polymerizable fluxing agent can be applied to the chip during the wafer stage of chip manufacture, often referred to as wafer-applied fluxing underfill, in which a plurality of chips are manufactured in one piece and later cut into individual chips. By pre-applying onto the wafer the fluxing agent/underfill sealant combination, the chip should only need to be placed on the substrate, with solder re-flow and underfill curing occurring to affix the chip thereto.
International Patent Publication No. WO 98/37134 refers to a no-flow underfill encapsulant for flip-chip technology. This encapsulant is based on epoxy resin(s), an anhydride hardener, an accelerator, a surfactant and a fluxing agent, and uses a viscosity-controlling agent, such as fumed silica, and a coupling agent. This encapsulant is reported to provide optimized flow and a curing reaction only after attaining the maximum solder bump reflow temperature of about 190-230° C.
U.S. Pat. No. 5,128,746 (Pennisi) describes a thermally curable adhesive having an acidic fluxing agent for use in reflow soldering an electrical component and a substrate. This adhesive reportedly removes oxide coatings on the metalization of the electrical component, and the adhesive at least partially cures when heated to solder reflow temperatures. The adhesive includes a thermoset resin, a fluxing agent, and a curing agent that reacts with and cures the thermoset resin when the thermally curable adhesive is heated. The exposure of the electrical component and substrate to a low pH environment caused by the acidic fluxing agent, however, can lead to corrosion of metal components.
U.S. Pat. Nos. 5,985,043 and 5,985,456 (Zhou et al.) disclose a thermally curable adhesive composition that includes a fluxing agent that also acts as an adhesive. The composition includes an unsaturated carboxylic acid fluxing agent and may further include a crosslinkable diluent, a source of free radical initiators, and a resin to react with remnant carboxylic acid moieties. However, the presence of the carboxylic acid functionality can result in the metalization of the electrical component being exposed to a low pH environment for extended periods, which can result in undesirable corrosion.
Accordingly, it would be desirable to provide a fluxing underfill composition that fluxes solderable surfaces, such as metal contacts, which the solder will electrically interconnect, and that possesses appropriate cure profiles for curing during the solder reflow cycle. It would be particularly desirable for the fluxing agent to not create an excessive acidic environment for prolonged periods of time, thereby preventing corrosion.